A liver cell-mediated mutagenesis system using primary cultures of rat liver cells to activate liver carcinogens and Chinese hamster V79 cells as the mutable cell has been developed. The liver carcinogens dimethylnitrosamine, diethylnitrosamine and aflatoxin B1 are mutagenic (cause ouabain resistance) to V79 cell when the primary liver cells are present. The non-carcinogenic analogues methyl-t-butylnitrosamine and aflatoxin G2 are inactive as mutagens. Such a system may be useful as a short-term assay for liver carcinogens. Primary cultures of liver cells can be cultured on a feeder layer of mouse embryo C3H/10T1/2 cells for 30-40 days. During this time the hepatocytes have morphological characteristics of liver cells in vivo and maintain a high viability, contain glycogen, and metabolize liver carcinogens. This new long-term method of culturing liver cells will allow studies of liver cell function after various treatments to be conducted in vitro.